Switching high-frequency electronic signals, such as electronic signals at ultra-high frequencies and beyond, presents substantially greater challenges than switching lower-frequency electronic signals. Such signals are carried by various types of transmission media such as coaxial cables and transmission lines to reduce signal losses. Whereas a single pair of contacts suffices to switch a low-frequency signal, complex switching arrangements are required to switch high-frequency signals in a manner that provides low signal losses, high isolation and appropriate termination impedances.
Relays are typically used in applications in which a high-frequency signal is switched in response to an electrical control signal. Relays, in which an electromagnetic coil actuates a pair of mechanical switching contacts, offer advantages of low capacitance, high isolation, low ON resistance and a high isolation between the control signal and the switched signal. When relays are used to switch high-frequency signals, multiple, commonly-controlled relays, each including its own electromagnetic coil, are often required to perform the desired switching function. The number of relays requires depends on the application.
FIG. 1 is a schematic diagram of an example 10 of a step attenuator for high-frequency signals. The step attenuator is composed of single-pole, double-throw relays 12 and 14, attenuator 16 and transmission lines 18, 19 and 20. Relay 12 is composed of electromagnetic coil 22 and a single-pole, double-throw switch having contacts 23, 24 and 25. Relay 14 is composed of electromagnetic coil 26 and a single-pole, double-throw switch having contacts 27, 28 and 29. Contact 23 of relay 12 is connected to input terminal 30. Contact 29 of relay 14 is connected to output terminal 27. Transmission line 18 interconnects contacts 24 and 27. Transmission line 19, attenuator 16 and transmission line 20 are connected in series between contacts 25 and 28.
In the switching state of step attenuator 10 shown in FIG. 1, no control signal is applied to the electromagnetic coils 22 and 26 of relays 12 and 14, respectively. In this switching state, input terminal 30 is connected to output terminal 32 via contacts 23 and 24 of relay 12, transmission line 18 and contacts 27 and 29 of relay 14. The step attenuator operates as a through line in this switching state.
A control voltage applied to electromagnetic coils 22 and 26 causes relays 12 and 14, respectively, to change to their other switching states. In this switching state, input terminal 30 is connected to one end of attenuator 16 via contacts 23 and 25 of relay 12 and transmission line 19. The other end of the attenuator is connected to output terminal 32 via transmission line 20 and contacts 28 and 29 of relay 14. In this switching state, step attenuator 10 operates as an attenuator, providing an attenuation determined by the attenuation provided by attenuator 16.
The circuit shown in FIG. 1 may also form the basis of a stepped delay circuit for a high-frequency signal. In such stepped delay circuit, a delay line (not shown) providing a predetermined delay is substituted for attenuator 16 in the circuit shown in FIG. 1.
FIG. 2 is a schematic diagram of an example 50 of an impedance-matched single-pole, double-throw switch for high-frequency signals. Switch 50 incorporates four single-pole, single-throw relays 51, 52, 53 and 54. Relays 51, 52, 53 and 54 are composed of contacts 61, 62, 63 and 64, respectively, and electromagnetic coils 71, 72, 73 and 74, respectively. Coaxial reed-relays may be used as relays 51-54. Switch 50 is additionally composed of termination resistors 56 and 58, signal connections 66, 76 and 78 and transmission lines 80, 82, 84, 86, 88 and 90.
Termination resistors 56 and 58 have a resistance equal to the characteristic impedance of the system in which switch 50 is to be used. The characteristic impedance is typically 50 xcexa9. Signal connections 66, 76 and 78 provide connections for the high-signal to be switched by switch 50. For example, signal connection 66 may be an input connection and signal connections 76 and 78 may be output connections. Alternatively, signal connections 76 and 78 may be input connections, and signal connection 66 an output connection.
Transmission lines 80 and 82 connect signal connection 66 to contacts 61 and 62 of relays 51 and 52, respectively. Transmission line 84 connects contacts 61 to signal connection 76. Transmission line 86, contacts 63 of relay 53 and termination resistor 56 are connected in series between contacts 61 and ground. Transmission line 88 connects contacts 62 to signal connection 78. Transmission line 90, contacts 64 of relay 54 and termination resistor 58 are connected in series between contacts 62 and ground.
In the switching state of impedance-matched, single-pole, double-throw switch 50 shown in FIG. 2, a control signal is applied to the electromagnetic coils 71 and 74 of relays 51 and 54, respectively, and no control signal is applied to the electromagnetic coils 72 and 73 of relays 52 and 53, respectively. In the examples for the relays shown, a control signal applied to the electromagnetic coil closes the switch contacts. In the switching state shown in FIG. 2, signal connection 66 is connected to signal connection 76 by transmission line 80, contacts 61 of relay 51 and transmission line 84. Signal connection 78 is connected to ground through transmission lines 88 and 90, switch contacts 64 of relay 54 and termination resistor 58. Thus, signal connection 66 and signal connection 76 are electrically connected while signal connection 78 is isolated from the other signal connections and is connected to ground through termination resistor 58.
In the alternative switching state of switch 50, a control signal is applied to the electromagnetic coils 72 and 73 of relays 52 and 53, respectively, and the control signal is removed from the electromagnetic coils 71 and 74 of relays 51 and 54, respectively. The change in control signals reverses the states of the switch contacts from that shown in FIG. 2. Signal connection 66 is connected to signal connection 78 and signal connection 76 is isolated from the other signal terminals and is connected to ground through termination resistor 56.
The relays used in the above-described circuits for high-frequency signals have a substantially larger volume than that of most other components used in modern high-frequency electronic circuits. The volume of a commercially-available transfer-type reed relay for high-frequency electronic signals is about 0.7 ml.
Test sets for testing high-frequency signals and for testing other apparatus that generate, process or receive high-frequency signals typically include many examples of the circuits shown in FIGS. 1 and 2. Such test sets may include embodiments of the above-described step attenuator having multiple attenuation steps, each of which requires two reed relays. Such test sets may additionally include several examples of the double-pole, double-throw impedance matched switch shown in FIG. 2 for selectively routing high-frequency signals in the test set. Accordingly, examples of such test sets that employ conventional switching circuits include a large number of reed relays. The aggregate volume of the reed relays and their associated drive circuits represents a substantial fraction of the volume of the test set.
Moreover, some commercially-available single-pole, double-throw switches incorporate coaxial reed relays to improve their impedance matching characteristics. However, the volume of a single-pole, double-throw switch incorporating coaxial reed relays is over 30 ml because the volume of the coaxial reed relays and their drive circuits is large. The volume of such switches is too large to allow many of them to be used in test sets and in other apparatus in which it is desired to reduce the overall volume of the apparatus.
The signal transmission properties of the reed relays used in the circuits described above are less than ideal, especially at higher frequencies. For example, the maximum frequency of the commercially-available transfer type RF reed relays used in step attenuator 10 shown in FIG. 1 can be as low as about 500 MHz. This is because of the large impedance mismatch between the reed relay and the transmission lines to which it is connected. Also, the attenuation of an input signal between signal connection 30 and signal connection 32 may be less than that provided by attenuator 16 due to coupling between transmission lines 19 and 20 and transmission line 18. This effect is worse when attenuator 16 provides a large attenuation and when the frequency of the signal is high.
The switching characteristics of switch 50 shown in FIG. 2 degrade at frequencies above those at which the wavelength is comparable with the size of the switch. Since the size of the switch is large, the switching characteristics degrade above a relatively low frequency. Commercially-available impedance matched, single-pole, double-throw switches based on the structure in FIG. 2 have a maximum frequency of about 1 GHz. A possible reason for this is that transmission lines 80 or 82 and 86 or 90 become open stubs on the internal transmission lines of the coaxial reed relays. The switching characteristics are degraded when the size of the transmission lines cannot be ignored in relation to the wavelength of the high-frequency signal.
Thus, what is needed for switching high-frequency signals is a switch device that is smaller in size than conventional switch devices. What is also needed is a switch device that does not suffer from the above-described performance shortcomings of conventional switch devices, especially at high signal frequencies. What is also needed is a switch device capable of switching signals having a substantially higher maximum frequency than conventional switch devices.
The invention provides a multi-pole, conductive liquid-based switch device that includes an elongate passage, a first cavity, a second cavity, at least four electrodes disposed along the length of the passage, channels that extend from the passage, non-conductive fluid located the cavities and conductive liquid located in the passage. The channels are one fewer in number than the electrodes and are interleaved with the electrodes along the length of the passage. The channels are numbered in order from one end of the passage. Odd-numbered ones of the channels extend to the first cavity while even-numbered ones of the channels extend to the second cavity.
A step attenuator or step delay device functionally similar to the step attenuator or step delay device shown in FIG. 1 can be made using a single multi-pole, conductive liquid-based switch device according to the invention with four poles. An impedance-matched, single-pole, double-throw switch for high-frequency signals similar to that shown in FIG. 2 can be made using a single multi-pole, conductive liquid-based switch device according to the invention with five poles. The volume of the step attenuator, the step delay device and the impedance-matched, single-pole, double-throw switch is substantially smaller than functionally-equivalent circuits fabricated using conventional reed-relays. Control signal routing is also simplified by only one switch device needing to be controlled.
Embodiments of the multi-pole, conductive liquid-based switch device according to the invention can include a ground plane and the passage and the electrodes can be structured as strip lines having a specific characteristic impedance that matches the characteristic impedance of the application in which the switch device is used. Signal losses and signal reflections are therefore smaller than with conventional reed-relays.